In recent years, largely due to the increasing popularity of the Internet, there has been an abrupt expansion in the volume of communications. This has stimulated improved methods for increasing the capacity of communications channels. One such method is time division multiplexing (“TDM”), in which information from several communication signals can share the same transmission channel. To do so, the bits for each signal are assigned to individual time slots or time periods that repeat or rotate so quickly that each signal has enough of the time slots to transmit all of its own information. When the information stream is received, the individual information signals are then separated based upon the assignments of each signal to its own repeating time periods or slots. The term “time division” thus refers to the time being divided into these many discrete time periods.
Another technology for increasing the capacity of communications channels is wavelength division multiplexing (“WDM”). WDM assigns the information signals to various different wavelengths (or colors) that are then separated at the receiver to recover the individual signals.
For the highest communication capacity, TDM and WDM are combined. For example, a standard communications protocol using both TDM and WDM currently employs 160 WDM channels, each channel carrying a 10 gigabits per second “(Gbps”) TDM signal. All this information is then carried by a single fiber, which may be part of a fiber optical cable.
To maintain the quality of information transmission networks, it is important to be able to analyze such multiplexed signal waveforms. This requires measuring both the wavelength spectrum (“frequency domain”) and the signal modulation as a function of time (“time domain”). Typically, optical spectrum analyzers are used to measure the wavelength spectrum, and sampling oscilloscopes are used to measure the signal as a function of time.
Optical spectrum analyzers are typically constructed so that the input signal light is reflected by a diffraction grating that separates the individual multiplexed wavelengths from one another much the same way that a prism separates visible light into its various colors. After each individual wavelength is isolated, the wavelength of interest is directed to a detector, such as an opto-electrical conversion element, that converts the light into an electrical output. To examine the range of wavelengths present in the original signal, the wavelength that is extracted is then incremented. This is accomplished by rotating the diffraction grating so that the various wavelengths are presented, in turn, to the detector. In typical measurements of a WDM signal, an optical spectrum analyzer can accomplish such a spectral analysis in a time interval of approximately several milliseconds. This signal may then be recorded, displayed, or subjected to further processing as desired.
For measuring the information signal in the time domain, i.e., as a function of time, the light signal that is to be measured is detected by a similar opto-electrical conversion element and converted into an electrical signal. The electrical signal is passed to a sampling circuit that is controlled by a strobe circuit. The strobe circuit generates a repetitive, short duration strobe signal that instructs the sampling circuit to extract only the portion of the electrical signal that is present at each instant that the strobe signal is applied. The extracted signal then passes from the sampling circuit through an amplifier to an analog-to-digital (“A/D”) converter. The resulting digital signal may then be recorded, displayed, or subjected to further processing as desired.
Due to the nature of optical signals, and in particular the extremely high information rates contained in the signals, previous techniques for performing such measurements are undesirably limited. A principal limitation is that measurements in the frequency domain and measurements in the time domain are performed separately, so that the measured signal characteristics lack simultaneity. There are also limitations with respect to the ranges of wavelengths that can be satisfactorily measured, and limitations in the optical power available for measuring the light signal as it is being processed by the measuring equipment.
For example, a previous measurement technique employs a repetitively pulsed light source that combines with the optical signal for transmission through a nonlinear optical crystal. The nonlinear crystal converts the wavelength of the WDM signal during each short time that the sampling light pulse is turned on. The converted wavelength is then separately detected and analyzed. Unfortunately, this results in limitations in the wavelength range that can be effectively measured, and also results in significant reductions in the resulting optical power of the final converted optical signal. In addition, to achieve the required frequency conversion, such previous techniques often use a special pulse laser with a high peak optical output power and a short pulse duration. However, not all WDM signals require such expensive and powerful analytical tools, in part because the large amounts of information in such WDM signals may be distributed across a large number of separate wavelength divisions.
A need therefore remains for methods and apparatus for efficiently and economically measuring wave shapes and wavelengths of optical signals of different wavelengths in a WDM system, while retaining substantially the full signal strength of each individual optical signal that is being measured.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long been elusive.